This invention relates to an automatic braking system (ABS) and/or automatic skid control (ASC) system for motor vehicles.
A control system for motor vehicles of the generic type is disclosed, for example, in the ATZ article xe2x80x9cBrake System and Slip Control Systems of the New BMW 7-Seriesxe2x80x9d, January 1995, Pages 15, and on. This known control system has a sensor arrangement for sensing the four rotational wheel speeds of a motor vehicle. Based on the signals of this sensor arrangement, a locking tendency of the wheels during high braking maneuvers is recognized and is prevented by normal ABS control. In the case of the system known as xe2x80x9cSpecial ABSxe2x80x9d, an additional wheel-selective control of the brake pressure is permitted in addition to the normal ABS control. When the driver triggers partial braking during cornering, a pulsed pressure buildup or a reduction of the brake pressure takes place on the inside rear wheel, or on the inside front and rear wheels, by means of the existing actuators of an ABS/ASC control system. xe2x80x9cPartial brakingxe2x80x9d is a brake operation in which an ABS control is initiated at no more than one wheel of the powered axle or both wheels of the nonpowered axle.
In this known system, cornering is recognized when a specific lateral acceleration threshold is exceeded. For this purpose, the lateral acceleration is realistically determined from the driving speed and the signal of a steering angle sensor.
German Patent Document DE 195 10 104, which has not been published, discloses a system in which cornering is recognized by computing a first actual value proportional to the lateral acceleration of the rear axle, based on the difference of the rotational speeds of the rear wheels, and/or by computing a second actual value proportional to the lateral acceleration of the front axle, based on the difference of the rotational speeds of the front wheels. In this manner cornering can be recognized without need of additional sensor arrangements, such as steering angle sensors or lateral acceleration sensors, by making multifunctional use of existing rotational wheel speed sensors. As used herein, xe2x80x9ccornering recognitionxe2x80x9d covers not only the extent but, for example, also the type of a curve (right-hand curve, left-hand curve).
In known control systems, a certain value of lateral acceleration (or of a quantity proportional to the lateral acceleration) is used as a criterion to determine only the existence or non-existence of cornering. When cornering is detected, or when the defined value is exceeded, only cornering-related roll-stabilizing measure is implemented, regardless of the extent of the curve. Therefore, in this case, only a roll stabilizing environmental parameter of the vehicle is taken into account. However, experiments have demonstrated that, particularly at a high vehicle speed, parameters caused by the vehicle construction, such as torsional movements, may also result in unstable conditions, even when the lateral acceleration is low.
It is an object of the present invention to provide an improved control system of the type mentioned above, which prevents unstable conditions that result from parameters caused by vehicle construction, and which affect vehicle operation, even when the lateral acceleration is low.
This object is achieved according to the invention, by carrying out a symmetrical reduction of the brake pressure on both wheels of an axle, preferably the powered axle, whenever a quantity proportional to lateral acceleration is greater than a first lower threshold value. Prerequisites for such a symmetrical reduction of brake pressure on both wheels of an axle are the presence of at least a slight curve, a simultaneous uninterrupted brake operation in the form of a partial brakingxe2x80x94as defined abovexe2x80x94, and optionally also a vehicle speed which is above a given threshold of, for example, approximately 110 km/h.
The symmetrical reduction of the brake pressure on both wheels of the powered axle, while the higher xe2x80x9cpilotxe2x80x9d brake pressure set by the driver is maintained on the wheels of the nonpowered axle, stabilizes the vehicle by generating forces similar to those which are generated by an active countersteering of the wheels of the powered axle (in the experiment, of the rear axle).
The invention thus achieves an effective low-cost stabilization of the vehicle with respect to vehicle-internal parameters that generate instability.
In an advantageous embodiment of the invention, if the signal indicative of lateral acceleration changes signs (indicating a direct change of curve direction) while brake operation is maintained, a symmetrical reduction of the brake pressure is thereafter carried out on both wheels of an axle when the quantity proportional to the lateral acceleration is larger than a second lower threshold value, which is smaller than the first lower threshold value, but larger than zero. This embodiment is particularly effective in the case of a double curve, such as an immediate left/right cornering sequence. During immediately successive curve direction changes, the response behavior of the measure according to the invention for increasing stability with respect to the curve(s) following the first curve, becomes even more sensitive.
According to another advantageous embodiment of the invention, brake pressure is reduced asymmetrically. That is, the brake pressure on the wheel which is on the inside during cornering is reduced to a greater extent than the brake pressure on the wheel which is on the outside during such cornering, whenever the lateral acceleration exceeds a given upper threshold value which is greater than the lower threshold values.
In this case, it is assumed that for curves that are more pronounced than a certain threshold, the effects of the curve-related external parameters exceed the effects of the vehicle-construction-related internal parameters. Thus, the known curve-related stability increasing measures are modified.
It should be added that generally the reduction of the brake pressure is preferably implemented abruptly, but is eliminated gradually, as comfortably as possible.
These and other objects, features and advantages of the present invention will become more readily apparent from the following detailed description of the drawing wherein: